For reduction in weight of an automobile, efforts are advanced to increase the strength of a steel material used for an automobile body and to reduce the weight of steel material used. In a thin steel sheet widely used for the automobile, press formability thereof generally decreases with an increase in strength, making it difficult to manufacture a component having a complicated shape. For example, a highly processed portion fractures with a decrease in ductility, and springback becomes prominent to deteriorate dimensional accuracy. Accordingly, it is difficult to manufacture components by performing press-forming on a high-strength steel sheet, in particular, a steel sheet having a tensile strength of 980 MPa or more. It is easy to process the high-strength steel sheet not by press-forming but by roll-forming, but its application target is limited to a component having a uniform cross section in a longitudinal direction.
Methods called hot pressing intended to obtain high formability in the high-strength steel sheet are described in Patent Literatures 1 and 2. By the hot pressing, it is possible to form the high-strength steel sheet with high accuracy to obtain a high-strength hot-pressed steel sheet member.
On the other hand, the hot-pressed steel sheet member is required to be improved also in ductility. However, steel structure of the steel sheet obtained by the methods described in Patent Literatures 1 and 2 is substantially a martensite single phase, and thus it is difficult for the methods to improve in ductility.
High-strength hot-pressed steel sheet members intended to improve in ductility are described in Patent Literatures 3 and 4, but in these conventional hot-pressed steel sheet members, it has another problem of a decrease in toughness. The decrease in toughness causes a problem not only in the case of the use for an automobile but also in the case of the use for a machine structural component. Patent Literatures 5 and 6 each describe a technique Intended to improve a fatigue property, but even these have difficulty in obtaining sufficient ductility and toughness.